The present invention relates to an pickup for an optical disc reproducing system, and more particularly to a pickup where a light source emits different laser beams, each having a different wavelength for reproducing information on different optical discs, each having different recording densities.
In order to improve efficiency of recording and reproducing information on the optical disc, an optical disc having a high recording density is proposed. To read such a disc, an optical pickup having a second harmonics generating device or a laser device which emits a beam of a short wavelength such as green light or blue light has been developed.
The pickup having the laser device emitting the beam of the short wavelength reads information recorded in high recording density with accuracy.
Conventionally, a disc having information of a low recording density such as a laser disc or a CD is reproduced by a beam of a long wavelength such as infrared rays.
It is desirable that the pickup can read both the high density disc and the conventional disc.
However, the beam of the short wavelength is radiated on the disc at a small spot diameter for reproducing the information in the high recording density. Therefore, if the short wavelength of the blue light is used for the disc of the low recording density, the spot diameter is too small for the pit on the disc and a phase difference dependent on the depth of the pit is different in wavelength. Accordingly, the level of the reproduced signal is low and signal distortion is increased. Therefore, it is difficult to obtain a good reproduction.
In order to solve such a problem, there has been proposed to provide a pickup having two laser beam sources for emitting laser beams having different wavelengths.
In particular, if the second harmonics generating device (SHG) comprising a laser device and a resonator is used, beams of long wavelength and short wavelength can be obtained.
FIG. 3 shows a conventional pickup having a second harmonics generating device. The second harmonics generating device comprises a semiconductor laser device 1 for emitting a beam of infrared rays and a nonlinear optical resonator 2 for producing a blue beam of a half wavelength of the infrared rays. Laser beams of the infrared rays emitted from the laser device 1 are fed to the nonlinear resonator 2. The nonlinear resonator 2 operates to convert a part of the beams into a beam of a blue light having short wavelength. The beams of the infrared rays and the blue light are reflected on a beam splitter 3 where the beams are changed in a perpendicular direction. The beams from the beam splitter 3 are arranged to parallel beams through a collimator lens 4 and fed to an objective 5. The beams are focused on a recording surface of an optical disc 6 by the objective 5.
The reflected beams from the optical disc 6 are reflected on a dichroic mirror 7 passing through the objective 5, collimator lens 4 and beam splitter 3, where the beams are divided into short wavelength and long wavelength. The beams of the blue light of the short wavelength are focused on a photodetector 8 on which divided photo-sensors are provided. The beams of the infrared rays of the long wavelength are focused on a photodetector 9 on which divided photo-sensors are provided. Thus, the photodetector 8 detects information on a disc having a high recording density and the photodetector 9 detects information on a disc having a low recording density. In the photodetector, a focus error is detected by the astigmatic method and a tracking error is detected by a push-pull method. Thus, the pickup having compatibility is obtained.
In such a pickup, it is necessary to provide the collimator lens 4 and the objective 5 so as to have a diffraction limited performance at the two wavelengths. Namely, both lens must be arranged to have small chromatic aberration. However, it is difficult to make a lens by molding plastic to have diffraction limited performance for two wavelengths. Therefore the objective 5 must be made of three lenses. It is possible to use different collimator lenses in accordance with the wavelength of the beam. However, in order to cooperate with the objective 5, the collimator lens 4 becomes large in size. Accordingly, the lenses are complicated in construction, thereby increasing manufacturing cost, and an actuator for operating the objective 5 becomes large, which causes an increase of power consumption.
Furthermore, since the photodetectors are individually provided, it is necessary to adjust positions thereof with accuracy when assembling, causing troublesome assembling.